Field of the Invention
This invention relates to solid state lighting packages, and more particularly to LED packages having encapsulants with thixotropic agents.
Description of the Related Art
Incandescent or filament-based lamps or bulbs are commonly used as light sources for both residential and commercial facilities. However, such lamps are highly inefficient light sources, with as much as 95% of the input energy lost, primarily in the form of heat or infrared energy. One common alternative to incandescent lamps, so-called compact fluorescent lamps (CFLs), are more effective at converting electricity into light but require the use of toxic materials which, along with its various compounds, can cause both chronic and acute poisoning and can lead to environmental pollution. One solution for improving the efficiency of lamps or bulbs is to use solid state devices such as light emitting diodes (LED or LEDs), rather than metal filaments, to produce light.
Light emitting diodes generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from various surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly can then be encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12.
FIG. 2 shows another embodiment of a conventional LED package 20 comprising one or more LED chips 22 mounted to a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflective cup 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflective cup 24 also provides mechanical protection to the LED chips 22. One or more wire bond connections 27 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflective cup 24 is typically attached to the carrier by means of a solder or epoxy bond.
FIG. 3 shows another embodiment conventional LED package 30 having an LED chip 32 mounted on a submount 34, similar to the LED package 20 shown in FIG. 2. In this embodiment, however, there is no reflective cup. In this embodiment, an encapsulant 36 is formed directly over the LED chip 32 and on the surface of the submount around the LED chip 32. Like the encapsulant in package 20, the encapsulant 36 can provide environmental and/or mechanical protection and can shape or alter the light emitting from the package.
Many encapsulants used in conventional LED packages utilize a thixotropic or thickening agent/material that can help the encapsulant maintain the desired shape. The packages shown in FIGS. 2 and 3, the encapsulant is in a hemispheric shape, and for dispensed lenses the thixotropic agent helps the encapsulant maintain a hemispheric shape particularly in the time between when the encapsulant is dispensed or molded, and when it is cured. One of the most common thixotropic agents is fumed silica that is commercially available from sources such as Cabot Corporation and Evonik Industries. Fumed silica is a relatively common material that is used in many different applications and has a very strong thickening effect. Fumed silica can be provided in particles that typically have a size in the range of 5-50 nanometers (nm). The particles can be non-porous and can have a surface area of 50-600 m2/g, with a density of around 2.2 g/cm3.
Many thixotropic agents can have an index of refraction that is different from the LED package encapsulant. For example, fumed silica can have an index of refraction of approximately 1.46, and can be mixed in a conventional encapsulant such as silicone which has an index or refraction of 1.51 or more. This difference in index of refraction between the thixotropic agent and the encapsulant can result in the encapsulant exhibiting scattering characteristics for the light passing through the encapsulant from the LED. This scattering not only gives the encapsulant a cloudy (i.e. not clear) appearance, but can reduce emission package efficiency by reducing the total light output from the package.